Some article components of gas turbine engines operate in a high-temperature environment produced by the combustion gases of the engine. Ever-higher operating temperatures are sought to improve the thermodynamic efficiency of the engine. In order to operate at high temperatures, the components are made of materials that retain the required mechanical properties at such temperatures. Even with the use of advanced materials such as superalloys, in some cases the capabilities of the materials are pushed to their limits, so that the operating temperature cannot be increased further.
Another approach is to create conditions in which the component operates cooler than it might otherwise, in the same high-temperature environment. One technique is to cool the component with a flow of cool air. Another technique is to coat the component with a ceramic thermal barrier coating that insulates the underlying metallic material from the hot combustion gases.
Yet another technique is to apply a heat-reflective coating, either a noble-metal layer or an optical coating, to the surface of the component to reflect an external heat load away from the component. That is, the heat energy radiated by the hot combustion gas and other hot components continues to be incident upon the coated component, but a part of the heat energy is reflected away from the component by the heat-reflective coating. The heat-reflective coating serves as a “heat mirror” to reflect heat from the surface in a manner somewhat analogous to a familiar light mirror that reflects light away from a surface. The noble-metal coating has limitations on its maximum use temperature due to diffusion and chemical interaction with some superalloys.
The heat-reflective optical coating typically has a multilayer structure with a number of coating layers. The thicknesses of the layers are tailored to reflect the various optical wavelengths that carry the heat energy in the 0.5-3 micrometer wavelength range. The materials of the optical coating are selected to withstand the required operating temperature. Radiated heat energy from flames is largely transmitted in the high-visible and near-infrared wavelength ranges having wavelengths of about 0.5-3 micrometers. The thicknesses of the layers are usually small, with each layer of a stack having 3-90 layers being on the order of about 0.005-25 micrometers thick. Techniques for designing such optical coatings are well known.
To apply an optical coating to the surface of an article, the article is placed into a deposition apparatus appropriate for the type of optical coating to be deposited. Typical application techniques include chemical vapor deposition (CVD) and physical vapor deposition (PVD), each of which requires a specialized deposition apparatus. The component article is placed into the deposition apparatus and processed to deposit the optical coating. CVD has limitations on equipment size, and PVD is a line-of-sight deposition technique that cannot be used to coat articles of complex geometries.
While this deposition approach is operable, the work leading to the present approach has identified some significant process limitations in practical applications. The deposition apparatus must be made sufficiently large to accommodate the article upon which the optical coating is to be applied, and in many cases special deposition apparatus must be built for very large articles. The capital and operating costs for the special deposition apparatus are high. To achieve the maximum production economies, the deposition apparatus is desirably made sufficiently large to accommodate a number of the articles. The article surface upon which the optical coating is to be applied may have an irregular geometry, so that it is difficult or impossible to deposit the optical coatings, with precisely defined layer compositions and layer thicknesses, over the entire article surface.
These limitations present challenges at the original manufacturing facility where the new-make article is made, but even greater challenges for repair operations. Most repair operations are performed at sites away from the original manufacturing facility, so that any improvements to the deposition apparatus used to deposit the optical coating must be duplicated, and process improvements implemented, at the remote repair site, in order to repair the optical coatings. The high costs of the available approaches for providing the coatings inhibit the installation of the required apparatus at the remote repair sites.
The result of these limitations is that, although optical coatings offer important benefits, their use is limited by the manufacturing difficulties that are encountered in both new-make and repair applications of the optical coatings. There is a need for an approach to applying optical coatings that may be used in a wide range of circumstances and is economically applicable for both new-make and repair articles. The present invention fulfills this need, and further provides related advantages.